Joanelizbay and everyone else, the max number of people per group is 20 which means 19 F8s. Only once have I exceeded that by one block to accommodate all who wanted to double. It was a nuisance, changing fabric needs right before starting and getting the ok from everyone. It was my idea and I regretted it immediately. This round, it's working out well so that those who want more blocks can get 19, not just 12. Not everyone wants to make a lot of blocks in a couple of months so group size remains at 20, which is 19 blocks.
I suppose it shouldn't be called "doubling up". It was a much better name when the group didn't have so many who wanted to do it so that we exceeded the max of 19.